Eukaryotic pathogens of many phylogenetic lineages can devastate crops via attack through the root structure, and the potential for damage is often not limited to a single year's crop, as many pathogens can live for years in infected soil and roots. For example, Armillaria root rot disease, caused by basidiomycete fungi from the Armillaria genus, is now the number one cause of peach tree death in the Southeastern United States.
Terrestrial oomycetes, the largest group of heterotrophic straminipiles, include a large number of plant pathogens and are believed to be among the most important plant pathogenic organisms that may be facultatively or obligately parasitic. These organisms include about 60 species of the genus Phytophthora and more than 100 species of the genus Pythium that are destructive plant pathogens. For instance, species of the oomycete genus Plasmopara are responsible for the downy mildews that affect grapes, lettuce, corn, cabbage, and many other crop plants. Species of Phytophthora can destroy eucalyptus, avocado, and pineapple, as well as other tropical crop plants. P. infestans, the Phytophthora species that causes late blight of potato, is well known in history. In one week in 1846, this disease wiped out almost the entire potato crop of Ireland.
Current management options for such plant pathogens are extremely limited. Since the pathogens can often survive for decades in infected root pieces or soil, crop rotation is of limited value. Furthermore, increasing environmental concern as well as expense in regard to the use of agricultural fumigants has made use of many previously common fumigants such as methyl bromide impractical.
Genetic manipulation approaches, and specifically creation of transgenic plants that exhibit increased resistance to pathogens is one approach for solving these problems. Difficulties exist with such approaches, however, as multiple transformations to protect a species from multiple pathogens is highly problematic with limited chances of successful propagation. Moreover, consumers are leery of produce that may contain the expression products of foreign DNA, and concern exists over the possible environmental impact should transgenic varieties of a plant escape into the wild, for instance through sexual reproduction mechanisms.
An anti-fungal gene has been discovered in a Chinese orchid, Gastrodia elata B1. F. falvida S. Chow. Specifically, the anti-fungal protein Gastrodia Anti-Fungal Protein (GAFP, also known as gastrodianin) was purified from the terminal corm of the orchid and shown to inhibit growth of Trichoderma in vitro (Hu, et al., ‘Isolation and partial characterization of an antifungal protein from Gastrodia elata corm’, Acta Bot. Yunnan 10:373-309, 1988). The protein was found to be a low molecular weight, monomeric lectin capable of binding both mannose and chitin (Xu, et al., ‘Purification and characterization of a novel anti-fungal protein from Gastrodia elata’, Plant Physio. 36:899-905). A gene expressing the protein was identified in 2001 by Wang, et al. (Gastrodianin-like mannose-binding proteins: a novel class of plant proteins with antifungal properties', Plant J. 25:651-661). (All three articles being incorporated herein by reference.) While gastrodianin has been shown to inhibit hyphal growth of several basidiomycete root rot pathogens including A. mellea, Rhizoctonia solani, and Ganoderma lucidum in vitro, the possibility of translating this anti-fungal activity to transgenic plants by incorporating the gene is not known to have been reported. Moreover, the activity of this protein when confronted with other pathogens, such as straminipile or metazoa pathogens, is not known to have been examined at all.
What is needed in the art are methods for preventing and controlling crop damage due to pathogens. In addition, what is needed in the art are crop plants that are more resistant to infection by such pathogens, and in particular, crop plants that can exhibit this increased resistance with no environmental hazard and no chance of release of a transgenic plant variety into the wild. Moreover, a single method that can simultaneously confer resistance to multiple pathogens, and in particular pathogens from multiple phylogenetic lineages, would be of great benefit.